The present invention relates to a height measuring apparatus for use in measuring the height of a small object such as a read frame or a ball grid array (BGA).
Among the steps of manufacturing a semiconductor device is a step of measuring the height of small and continuous structures, such as the leads of a lead frame or the ball grids of a ball grid array to be mounted on a bare chip. In recent years there is the trend that semiconductor components have higher integration density and a larger size. This trend is accompanied by the demand for a height measuring apparatus that can make measurement at more points at higher speed.
Hitherto known as a height measuring apparatus is an apparatus that can measure a height at a point of measurement point by utilizing out-focus signals. This apparatus applies a light beam through an objective lens to the point of measurement. A beam splitter splits the light beam reflected from the point of measurement into two parts. One first part of the beam is applied through the first diaphragm arranged in front of a beam-condensing point. The first light-receiving element detects this part of the beam. The other part of the beam is applied through the second diaphragm located at the back of the beam-condensing point. The second light-receiving element detects the other part of the beam.
Thus structured, the apparatus can generates a plus/minus signal and an out-focus signal from the output signals of the first and second light-receiving elements. The magnitude of the plus/minus signal is zero (0) at the beam-condensing point, and has a positive or negative value at a point near the beam-condensing point. The out-focus signal has a magnitude that is proportional to the out-focus distance. Hence, the height at each point on an object can be obtained by moving the objective lens in the direction of the height until the magnitude of the out-focus signal becomes zero, and by measuring the height of the objective lens.
With the height measuring apparatus it is necessary to move the objective lens (or each point of measurement) up and down to measure the height at each point of measurement. Inevitably it would take a very long time to finish measuring the height at a number of measurement points, as is required in the electronic components developed recently. This may results in a decrease in the efficiency of manufacturing the electronic components.
The present invention has been made in view of the foregoing. Its object is to provide a height measuring apparatus which lessens the burden on the operator and which can achieve high operating efficiency.
The more detailed object of this invention is to provide a height measuring apparatus in which it is unnecessary to move the objective lens (or the point of measurement) up or down every time height is measured at one point.
Another object of the invention is to provide a height measuring apparatus, which has an optical system capable of reducing the cause of measurement error. In order to attain the objects mentioned above, a height measuring apparatus according to a first aspect of the invention comprises: an illumination optical system for sequentially illuminating a surface of an object; an imaging optical system having an optical axis spaced by a predetermined distance from an optical axis of all light beam reflected from the object, and designed to converge a part of the light beam along this optical axis; optical position-detecting device arranged in a converging plane of the imaging optical system, for detecting a light spot which changes in position in accordance with the height of the object; and height calculator for generating a spot position signal from a light-intensity signal supplied from the optical position-detecting device and for calculating the height of the surface of the object from the spot position signal.
With this arrangement it is possible to measure the height of the surface of the object on the basis of triangulation and, hence, to detect the height of the surface of the object continuously without the necessity of moving the stage or the like every time measurement is made at one point.
According to a second aspect of the invention, there is provided a height measuring apparatus of the type according to the first aspect, characterized in that the illumination optical system further comprises: a light source for emitting measuring light; an objective lens arranged, opposing the object, for converging the measuring light on the object; and a light-scanning mirror provided in a pupil plane or conjugate plane of the objective lens, capable of rotating, for applying the measuring light to scan the object, and the imaging optical system comprises: an optical-path branching optical system for separating a light beam reflected by the object, from the measuring light; a pupil relay lens system for projecting a pupil plane of the objective lens on a pupil projection plane; and an off-axis imaging lens arranged on the pupil projection plane on which the pupil has been projected by the pupil relay lens system, spaced from an optical axis of the pupil projection plane, and designed to focusing on the optical position-detecting device a part of the light beam extracted by the optical-path branching optical system, which passes a part of the pupil of the objective lens.
With this arrangement it is unnecessary to move the stage every measurement is made at one point, since the light-scanning mirror is moved. Thus, the height of the surface of the object can be detected continuously.
In this arrangement, the optical axis of the light beam output from the optical-path branching optical system can remain constant. An optical system having a small number of factors that may cause measurement errors can be constituted.
According to a third aspect of the invention, there is provided a height measuring apparatus of the type according to the first aspect, which further comprises: measurement possible/impossible determining device for determining whether it is possible to perform height measurement, on the basis of the light-intensity signal supplied from the optical position-detecting device.
In this arrangement, measurement possible/impossible determining means can determine whether height measurement can be performed at each measurement position. Hence, only reliable results of measurement can be output.
According to a fourth aspect of the invention, there is provided a height measuring apparatus of the type according to the first aspect, characterized in that a focusing-direction drive mechanism is provided for moving the imaging optical system and the object away from each other, thereby to control focusing/defocusing of illumination light with respect to the surface of the object, and that the height calculating section controls the focusing-direction drive mechanism in accordance with a height of a prescribed reference part of the object, thereby to control the control focusing/defocusing.
With this arrangement it is possible to measure the surface of the object in a substantially focused state, regardless of the difference in shape between the stage, the tray or the like that carries the object. Hence, all objects can be measured in substantially the same state, and the accuracy of measurement can increase.
If the object has a rough surface, measurement can be performed in a certain defocusing state.
According to a fifth aspect of this invention, there is provided a height measuring apparatus of the type according to the first aspect, which is characterized in that:
the other optical position-detecting device comprises a photoacoustic element for continuously changing a diffraction angle and a light-detecting element for detecting a light beam diffracted by the photoacoustic element, and
the height calculating section calculates a change in the position of the spot on the basis of a signal generated by the light-detecting element and the diffraction angle detected by the photoacoustic element.
With this arrangement, the photodetecting element can be a photodiode having a high response speed. Hence, the height measurement can be effected at high speed.
According to a sixth aspect of the invention, there is provided a height measuring apparatus of the type according to the second aspect, characterized in that the light-scanning mirror is arranged between the optical-path branching optical system and the objective lens, an imaging lens is provided for focusing a light beam reflected by the object and branched by the optical-path branching optical system, on an intermediate image surface, an another objective lens of infinity design is provided for imaging again the image of the object formed near the intermediate image plane, and the off-axis imaging lens is arranged at the back focal plane of the objective lens of infinity design.
According to a seventh aspect of this invention, there is provided a height measuring apparatus of the type according to the first aspect, characterized in that:
the imaging optical system and the optical position-detecting device are arranged symmetrical to each other with respect to the optical axis of all light beams reflected by the object,
the illumination optical system has:
a plurality of light sources;
a lens for projecting an image of each light source on the object; and
light-source image scanning device for turning on the plurality of light sources sequentially, thereby to scan the image of the light source on the surface of the object in a predetermined direction, and
the height calculator calculates the height z of the surface of the object from a position x at which the image of the light source is projected on the surface of the object and positions xcex41 and xcex41 of light spots detected by the pair of position detecting elements.
In this arrangement, the light-source image scanning device for turning on the plurality of light sources sequentially. The height of the surface of the object can therefore be measured on the basis of the principle of triangulation. The stage need not be moved every time measurement is made at one point. The height of the object surface can be performed continuously.